17.2 Mirrors, Lenses, and Images Objects are real physical things that give off or reflect light rays. Images are “pictures” of objects that are formed in space where light rays meet.

17.2 Mirrors, Lenses, and Images The most common image we see every day is our own reflection in a mirror. The image in a mirror is called a virtual image because the light rays do not actually come together. The virtual image in a flat mirror is created by the eye and brain.

17.2 Mirrors, Lenses, and Images Light rays that enter a converging lens parallel to its axis bend to meet at a point called the focal point. The distance from the center of the lens to the focal point is called the focal length. The optical axis usually goes through the center of the lens.

A converging lens bends an incident light ray parallel to the optical axis toward the focal point. A diverging lens bends an incident light ray parallel to the axis outward, away from the focal point

17.2 The image formed by a lens A lens can form a virtual image just as a mirror does. Rays from the same point on an object are bent by the lens so that they appear to come from a much larger object.

17.2 The image formed by a lens A converging lens can also form a real image. In a real image, light rays from the object actually come back together.

17.2 Drawing ray diagrams A ray diagram is the best way to understand what type of image is formed by a lens, and whether the image is magnified or inverted. These three rays follow the rules for how light rays are bent by the lens: A light ray passing through the center of the lens is not deflected at all (A). A light ray parallel to the axis passes through the far focal point (B). A light ray passing through the near focal point emerges parallel to the axis (C).

17.3 Thin lens formula 1 + 1 = 1 do di df The thin lens formula is a mathematical way to do ray diagrams with algebra instead of drawing lines on graph paper. 1 + 1 = 1 do di df Object distance (cm) focal length (cm) Image distance (cm)

17.3 Use the thin lens formula 1) You are asked for image distance. 2) You are given the focal length and object distance. 3) The thin lens formula applies: 1/di = 1/f — 1/do 4) Solve for di 1/di = 1/4 — 1/6 1/di = 3/12 — 2/12 = 1/12 di = 12 cm The image forms 12 cm to the right of the lens. Calculate the location of the image if the object is 6 cm in front of a converging lens with a focal length of 4 cm.